This invention is related to a corrosion inhibitor for metal surfaces. More specifically, the present invention is related to a corrosion inhibitor having particular utility in acid gas scrubbing systems.
The sweetening of natural and synthetic gases has been practiced for many years. Typically this has involved the removal of acidic compounds such as CO.sub.2, H.sub.2 S and COS by absorption of the acid gases in a liquid absorbent medium. Various aqueous, absorbing or scrubbing solutions have been utilized to remove the acidic components including alkanolamines, sulfolane (2,3,4,5 tetrahydrothiophene-1,1 dioxide), 2,3,4,5 tetrahydrothiophene-1,1, dioxide with diisopropanol amine, and potassium carbonate solutions. Each of these systems experiences corrosion which may be attributable to one or more of the following: decomposition of the scrubbing solution; reaction of the acidic components of the gas and the absorbent; and direct attack of the acidic components in the gases. This corrosion may occur throughout the entire gas treating system contacted by scrubbing solution and acid gas. It has been found that this corrosion may be particularly severe in systems where the concentration of H.sub.2 S in the inlet gas is relatively low, i.e. less than 0.3 vol.%, either because the gas source was relatively low in H.sub.2 S or because previous processing had reduced the H.sub.2 S content of the gas. It is believed that H.sub.2 S promotes the formation of a protective sulfide coating on the exposed surfaces thereby inhibiting corrosion.
U.S. Pat. No. 3,311,529 discloses the use of a molybdate coating and a low solubility (i.e. 3-300 parts per million) antimony compound coating between dissimilar metals to inhibit galvanic activity and corrosion. The preferred antimony compound is antimony trioxide. This patent does not disclose the use of antimony compounds having high solubility and does not disclose alkali metal antimony salts. The specifically disclosed molybdates are those having a low solubility, such as barium or strontium molybdate, which can be incorporated into a paint or primer. This patent does not disclose or suggest a corrosion inhibitor having a relatively high solubility which could be added to the scrubbing solution of an acid gas treating system.
Several corrosion inhibitors have been directed at decreasing the corrosion rate of gas treating systems such as those previously described. U.S. Pat. No. 3,951,844 is directed at a corrosion inhibitor composition for ferrous metals in an acid gas treating plant utilizing an aqueous potassium carbonate scrubbing solution. The corrosion inhibitor comprises a mixture of a vanadium compound capable of ionizing in the aqueous potassium carbonate solution to provide pentavalent vanadium ions and an antimony compound which is at least partially soluble in the aqueous potassium carbonate solution. Among the preferred compounds are the antimony compounds such as alkali metal antimony tartrates, and other derivatives of polyhydroxyorganic acids, where the carboxylic acid molecule has from about two to about six carbon atoms. Specifically preferred compounds are potassium antimony tartrate and sodium antimony tartrate. Other compounds disclosed include oxides of antimony, alkali metal meta-antimonites, pyroantimonites, meta-antimonates, and antimony sulfates. U.S. Pat. No. 3,808,140 also discloses the combination of an antimony compound and a vanadium compound as a corrosion inhibitor. Preferred antimony compounds include alkali metal antimony tartrates, alkali metal antimony gluconates and other antimony derivatives of polyhydroxy organic acids. U.S. Pat. No. 3,959,170 also discloses the use of antimony and vanadium compounds as corrosion inhibitors. Preferred compounds include alkali metal tartrates. A specifically disclosed corrosion inhibitor is potassium antimony tartrate and sodium metavanadate. U.S. Pat. No. 2,869,978 also discloses the use of antimony compounds for corrosion inhibitors in acid gas systems. Aqueous sodium antimony tartrate and aqueous potassium antimony tartrate are disclosed as being effective in amine scrubbing solutions used to remove carbon dioxide and hydrogen sulfide. While vanadium compounds may be effective as corrosion inhibitors, an oxidizing agent is required to maintain the vanadium as an effective corrosion inhibitor. Frequently this is accomplished by air injection or by the use of an oxidant in the solution. In a system utilizing hindered amines, air injection or use of an oxidant might result in the oxidation of the amine activator and/or the co-solvent. This is undesirable, particularly in view of the relatively high cost of the amine activators and co-solvents. U.S. Pat. No. 3,087,778 also discloses the use of soluble compounds of antimony, arsenic, bismuth and phosphorus as corrosion inhibitors for the hot carbonate gas scrubbing process.
Japanese Pat. Nos. 5-3053-539 and 5-3053-540 both disclose the use of potassium antimony tartrate to inhibit the corrosion of steel in a scrubbing system used to remove CO.sub.2 from gas. These patents also utilize aminocarboxylic acid, its alkali metal salt or an ethylene polyamine. None of the above-noted patents discloses the combination of the antimony compound with a molybdenum salt.
Molybdenum compounds also have been used as corrosion inhibitors. In the Journal of Chemical Technology and Biotechnology, Volume 29, pages 619-628 (1979) Armour and Robitaille disclose the use of sodium molybdate as a corrosion inhibitor for cooling water systems. Japanese Pat. Nos. 5-3075-139 and 7-4030-626 also disclose use of sodium molybdate as a metal corrosion inhibitor in aqueous systems. U.S. Pat. No. 4,138,353 discloses the use of molybdates with citric acid and/or its alkali metal salts for corrosion inhibitors of aqueous systems. U.S. Pat. No. 4,132,667 discloses the use of sodium zinc molybdate as a corrosion inhibiting pigment. None of these publications discloses or suggests adding an alkali metal antimony tartrate to the solution containing the molybdate.
Accordingly, it is desirable to formulate a soluble corrosion inhibitor which provides effective corrosion protection, does not promote amine degradation, and does not require the presence of oxygen or other oxidizing agents for its performance.
It also is desirable to provide a corrosion inhibitor which is compatible with the solution, does not promote foaming and which is stable at the scrubbing solution operating temperatures.
It also is desirable to provide a corrosion inhibitor which does not interfere with the mass transfer rate and the absorption ability of the scrubbing solution.
It has been found that a combination corrosion inhibitor comprising an antimony salt and a molybdenum salt exhibits improved corrosion protection while not adversely affecting the effectiveness of the scrubbing solution.